This invention relates to textile manufacturing. More specifically, this invention relates to a computer numerically controlled machine for stitching a large fiber-reinforced preform having variable thickness.
Large aircraft structures such as wing covers are now being fabricated from textile composites. The textile composites are attractive because of their potential for lowering the cost of fabricating the large aircraft structures. Cutting pieces of fabric and stitching the fabric pieces together has the potential of being less expensive than cutting sheets of aluminum, drilling holes in the aluminum sheets, removing excess metal and assembling metal fasteners.
The wing cover can be made from a carbon-fiber textile composite. Sheets of knitted carbon-fiber fabric are cut out into pieces having specified sizes and shapes. Fabric pieces having the size and shape of a wing are laid out first. Several of these pieces are stacked to form the wing cover. Additional pieces are stacked to provide added strength in high stress areas. After the fabric pieces are arranged in their proper positions, the pieces are stitched together to form a wing preform. Secondary details such as spar caps, stringers and intercostals are then stitched onto the wing preform. Such a wing preform might have a thickness varying between 0.05 inches and 1.5 inches. The wing preform is quite large, and its surface is very complex, usually a compound contoured three-dimensional surface.
The wing preform is transferred to an outer mold line tool that has the shape of an aircraft wing. Prior to the transfer, a surface of the outer mold line tool is covered with a congealed epoxy-resin. The tool and the stitched preform are placed in an autoclave. Under high pressure and temperature, the resin is infused into the stitched preform and cured. This results in a cured wing cover that is ready for assembly into a final wing structure.
For textile composite technology to be successful, two barriers must be overcome: cost and damage tolerance. Damage tolerance appears to have been hurdled. Closely-spaced stitches on the wing preform provide sufficient damage tolerance because the stitches provide a third continuous column of material.
Cost continues to be the problem. An exceedingly large number of stitches must be made on the wing preform. A computer numerically controlled ("CNC") stitching machine might be required to make eight to ten stitches per inch, in rows that might be spaced 0.1 inches to 0.5 inches apart, over a surface that might be longer than forty feet and wider than eight feet. The total number of stitching points on the wing preform might exceed 1.5 million. Moreover, the stitches must be of high quality.
None of the conventional stitching machines are suited for making stitches in a surface as complex as a compound, contoured three-dimensional surface. None of the conventional stitching machines allow such a large preform to be fed to stitching heads. None of the conventional stitching machines make the quality of stitch required for aircraft preforms--a loose, modified Federal 301 Type Lock Stitch that is locked on the bottom surface of the preform. None of the conventional stitching machines are fully automated to perform stitching functions such as adjusting thread path geometry, adjusting thread tension, thread cutting, thread dragging and needle cooling. Machine operators would be required to constantly stop the machine, perform functions such as cutting thread and adjusting thread tension, and then restart the stitching machine. The manual labor of the machine operator adds to the cost of manufacturing and slows the time to manufacture the preform. Time and resources expended correcting operator errors also adds to the cost of manufacturing.
Based on the foregoing, it can be appreciated that there exists a need for a stitching machine that can make high-quality stitches in large preforms, especially preforms having compound, complex three-dimensional surfaces. There also exists a need for a stitching machine that can make the stitches quickly. There also exists a need for a stitching machine that requires little to no operator intervention.